a) Field of the Invention
The present invention relates to semiconductor devices, and more particularly to semiconductor devices using a group III-V compound semiconductor crystal in a charge carrier transport region or a region adjacent thereto.
b) Description of the Related Art
Group III-V semiconductors have various excellent properties and are used in optical semiconductor devices having a direct transition (direct gap) structure, in high speed (high frequency) semiconductor devices having high charge carrier mobility characteristics, and in other semiconductor devices.
Band gap, lattice constant, refraction factor and similar characteristics of mixed crystal semiconductors consisting of two or more group III-V semiconductors can be controlled, and mixed crystal semiconductors may be used to provide semiconductor regions of desired properties. For example, various mixed crystal semiconductors are used in semiconductor laser devices in order to concentrate carriers in the active radiative region, confine radiated light within a predetermined region, and emit light having a desired wavelength.
With recent rapid progress of crystal growth technology, there have been developed molecular beam epitaxy (MBE) procedures allowing the control of film thickness and composition at the molecular level, organo-metallic vapor phase epitaxy (MOVPE) procedures, atomic layer epitaxy (ALE) procedures, and the like.
A combination of super-fine crystal growth techniques and band engineering using mixed crystals for the arbitrary control of energy ban structure, lattice constant, and the like, allows for the realization of desired electrical and optical properties in semiconductor structures. For example, it is possible to realize a quantum well structure (superlattice structure) through modulation doping, enabling separation of impurity atoms from an excessive charge carrier.
GaAs based material is widely used in field effect transistors (FETs) because this material provides substrates of a large diameter and good quality and has good lattice matching with other materials used for forming hetero-junctions. For example, high electron mobility transistors (HEMT) and the like are produced using a GaAs layer on a GaAs substrate as the electron transport layer and using an AlGaAs layer as the electron supply layer.
AlGaAs has a wider band gap and a higher conduction band energy level than GaAs. Therefore, electrons move from an n-type AlGaAs layer to an adjacent GaAs layer. Even with a GaAs layer made of a high purity (i-type) and high mobility crystal, a great number of electrons can be supplied from the AlGaAs. These electrons are trapped by a potential well formed at the hetero-junction interface, and generate a two-dimensional electron gas (2 DEG) providing high speed operation.
It is usual to form FET channels using a high electron mobility GaAs, as discussed above.
A combination of GaAs and AlGaAs does not have such a large discontinuity of the conduction band edge (about 60% of the forbidden band). Moreover, the AlGaAs layer cannot be doped at high concentrations. As a result, 2DEG concentration is low in the GaAs channel.
It is preferable to form the channel layer using a material with higher mobility, if possible, in order to develop high speed devices. Mixed crystal InGaAs semiconductors made of InAs and GaAs satisfy this condition. The channel (electron transport layer) of InGaAs provides a faster carrier drift speed than GaAs.
If an InAlAs electron supply layer is used for forming a potential barrier relative to the InGaAs channel (electron transport layer), a larger band edge discontinuity value of the conduction band and high concentration doping are possible providing the expected superior properties to a GaAs channel (electron transport layer).
However, use of mixed crystal semiconductors such as InGaAs as the channel results in the phenomenon specific to mixed crystals in terms of carrier transport.
Hot electron transistors (HET) has been developed in which hot electrons moving at high speed are used as the carrier HETs have an emitter barrier region between the emitter region and base region. Carrier electrons pass through the emitter barrier region by the tunneling effect, and electrons injected to the base region move at high speed.
A collector barrier region is also formed between the base region and the collector region. Base current stop page results when a bias voltage is supplied between the base electrode and collector electrode.
Such hot electron transistors are formed in the following manner. For example, an n-type GaAs region to be used as the collector region is formed on a seminsulating GaAs substrate. On this GaAs region, a non-doped AlGaAs region to be used as the collector barrier region is formed. On this AlGaAs region, a GaAs region to be used as the base region is formed. On this GaAs region, a non-doped AlGaAs region to be used as the emitter barrier region is formed. On this AlGaAs region, an n-type GaAs region is formed as the emitter region. With such a hot electron transistor, when a positive voltage relative to the emitter voltage is applied to the base while maintaining the HET at 77.degree. K. for example, electrons of the emitter region pass through the emitter barrier between the emitter and base by the tunneling effect, and move in the base region as hot electrons. The hot electrons then pass through the collector barrier between the base and collector. Most of the hot electrons thus reach the collector. Very high speed transistor operation is therefore possible by controlling the base voltage.
Such a hot electron transistor has a GaAs/AlGaAs hetero-junction structure which is associated with the following phenomenon. Namely, because in the conduction band of GaAs, a valley (L point) of about 1.7 eV band gap is present above a valley (T point) with the minimum band gap of about 1.53 eV, some hot electrons having an energy level higher than necessary for passing through the collector barrier between the base and collector regions may move into the high level valley which lowers the hot electron speed.
In order to solve this problem, hot electron transistors have been proposed wherein InGaAs having a level difference of 0.5 eV between the T and L valleys in the conduction band greater than GaAs is used as a base layer or the like.
In InGaAs mixed crystals, In and Ga atoms assume random positions in the crystal lattices as the group III elements, resulting in irregularities in the lattice potential. If InGaAs is used as the base region in a hot electron transistor, the probability of scattering hot electrons increases because of these crystal lattice irregularities, thus reducing the collector transfer efficiency and limiting the amplification factor and other parameters.
FIG. 5A is a schematic diagram of the lattice structure of a mixed crystal material grown using conventional techniques. In FIG. 5A, the triangle symbols represent In atoms, the square symbols represent Ga atoms, and the circle symbols As atoms.
In InGaAs mixed crystal semiconductors, although As atoms take positions in the lattice structure as group V atoms, the positions taken by In and Ga Atoms as group III atoms is not definite.
The radius of an In atom is greater than that of a Ga atom. Therefore, the influence of the In atoms in the lattice structure depends upon whether a Ga atom or an In atom takes a particular position as a group III atom. This influence disturbs the regularity of the crystal lattices, resulting in scattering of carrier motion. This indefiniteness of the positions of the group III atoms results in electrically neutral alloy scattering.
In the above description, the indefiniteness of the positions of group III atoms has been explained. Similar alloy scattering also occurs in the case of the indefiniteness of the positions of group V atoms. If the positions of both group III and V atoms are indefinite, additive alloy scatterings occur.
Known scattering mechanisms which limit carrier mobility include phonon scattering, ionized impurity scattering, alloy scattering, and the like. Compound semiconductors such as GaAs are also associated with phonon scattering and ionized impurity scattering.
Alloy scattering is a scattering mechanism specific to mixed crystals. Alloy scattering is essentially less dependent upon temperature than are phonon scattering, ionized impurity scattering, and the like. Alloy scattering thus becomes conspicuous at lower temperatures.
FIG. 5B compares the influence of temperature on electron mobility caused by scattering in an In.sub.0.5 Ga.sub.0.5 As crystal as shown in FIG. 5A with the influence of temperature on electron mobility in an ordered mixed crystal of (InAs).sub.1 (GaAs).sub.1 having layer regularity (layer orderliness). The difference is the result of alloy scattering in the FIG. 5A structure. Although the relative amounts of In, As, and Ga in each of these mixed crystals are the same, the ordered mixed crystal (InAs).sub.1 (GaAs).sub.1 has a higher electron mobility because of the absence of alloy scattering. Optical phonon scattering becomes strong at a temperature of about 100.degree. K. and higher. Electron mobility in an ordered mixed (InAs).sub.1 (GaAs).sub.1 crystal is governed almost entirely by optical phonon scattering at temperatures near room temperature, and approaches the mobility of electrons in irregular or disordered mixed crystals.
Semiconductor devices using group III-V compound semiconductor mixed crystal layers as charge carrier transport layers do not exhibit performance enhancement to the degree expected, because of the decrease in electron mobility as a result of the alloy scattering mechanism.
Approaches for providing layer regularity have been proposed. After the report of (AlAs).sub.III (GaAs).sub.II monomolecular layer growth by MBE in 1976 (A. C. Gossard et al., Appl. Phys. Lett. 29 (1976) 323), an (InAs).sub.1 (GaAs).sub.1 monolayer produced by MOCVD was reported by Fukui and an (AlAs).sub.III (GaAs).sub.II monolayer was reported by Ishibashi (T. Fukui and H. Saito: Jpn. J. Appl. Phys. 23 (1984) L521; Ishibashi, et al., Lectures of the Japan Society of Applied Physics, Autumn, 1984, 13a-S-10).
All these approaches used a substrate surface in the (001) plane for layer regularity superlattice growth. An (InAs).sub.1 (GaAs).sub.1 mixed crystal having layer regularity formed on an InP (001) plane was actually observed by electron beam diffraction, x-ray diffraction, or the like (Fukui: Study Report No. 408, Divisions: Solid State Physics and Application, the Japan Society of Applied Physics, (1985) 13).
P. M. Petroff et al. proposed to control the lateral composition by using monolayer steps formed on an off-angle (slightly tilted) substrate having its growth surface tilted at a small angle from the crystal plane of low index (P. M. Petroff et al., Appl. Phys. Lett., 45 (1984) 620), and applied it to monolayer growth of (AlAs).sub.III (GaAs).sub.II by MBE. The two-dimensional composition control of (AlAs).sub.1/2 (GaAs).sub.1/2 by MOCVD was first reported by Fukui et al. (Fukui et al.: Study Report No. 418, Divisions: Solid State Physics and Application, the Japan Society of Applied Physics, (1987) 13).
In a quantum well structure, a charge carrier can be localized at quantum levels. The localization of quantum levels is improved as the order of the quantum well is raised. A two-dimensional confinement structure for a one-dimensional quantum well is called a quantum wire (one-dimensional electron series), and a three-dimensional confinement structure is called a quantum box (zero-dimensional electron series).
FIGS. 3A to 3C show the quantumized states and the relationships between quantumized energies and state densities p(E). FIG. 3A illustrates one-dimensional quantumization, FIG. 3B illustrates two-dimensional quantumization, and FIG. 3C illustrates three-dimensional quantumization.
For bulk crystals, the parabolic state density function changes to one having a stepwise shape, a sawtooth shape, or a linear shape as shown in FIGS. 3A to 3C, respectively for the one-, two-, or three-dimensional confinement structures.
It is known that the one-and two- dimensional quantumization structures (quantum well structure and quantum wire structure) provide high mobility for charge carrier drift in the non-confined direction because of the suppression of impurity scattering and the simplicity of scattering mechanisms. Thus, these structures are practically applied to some high performance transistors such as HEMT.
It has been pointed out that such quantum size effects have a great influence on the optical performances of group III-V compound semiconductors. For example, high order confinement lasers are expected to have the following desirable characteristics (T. Takahashi and Y. Arakawa, Optoelectronics 3, (1988), pp. 155-163):
(1) low threshold value; PA1 (2) lower temperature dependence for threshold currents; PA1 (3) high resonance frequency of relaxation oscillation, which is an upper limit frequency capable of direct modulation; PA1 (4) narrower oscillation spectrum width; and the like. PA1 (a) Combination of lithography and etching PA1 A one-dimensional quantum well structure is first formed, and unnecessary regions are patterned and etched. However, during these processes, the crystal structure at the etched edges may be damaged. PA1 (b) Combination of lithography and selective growth PA1 The confinement structure is selectively grown at patterned regions. However, the size of the structure is governed by lithography precision characteristics so that the minimum size is in the order of 50 nm at best. PA1 (c) Fine region growth using surface steps PA1 The lateral composition is controlled using crystal surface steps formed on a substrate having a surface which is slightly tilted from the (001) plane. This method was first proposed by P. M. Petroff et al., and completed by Fukui et al. providing a (AlAs).sub.1/2 (GaAs).sub.1/2 structure by MOVPE (P. M. Petroff et al., Appl. Phys. Lett., 45 (1984) 620; Takashi Fukui et al., Study Report No. 418, Divisions: Solid State Physics and Application, the Japan Society of Applied Physics, (1987) 13).
Methods for forming such high order confinement structures have been tried conventionally. Some of these methods and their shortcomings are described below.
FIGS. 4A to 4C schematic diagrams showing directional crystal growth on a substrate provided with steps.
As shown in FIGS. 4A, AlAs is grown on a GaAs substrate having a growth surface that is slightly tilted from the (001) plane. AlAs is first grown laterally from the stepped area. Although atoms on the flat plane of the substrate are attached to the substrate only at their bottoms, atoms at the stepped area of the substrate are attached to the substrate at both their bottoms and sides, thus stabilizing the atom layer attachment. After growing the AlAs layer portion across about one half of the step, a GaAs layer portion is grown on the remaining half of the step as shown in FIG. 4B. After the step is covered with a monomolecular layer, AlAs is again grown. Such a process is repeated to obtain the structure shown in FIG. 4C.
Through the use of the foregoing process damage is eliminated. Moreover, the size of the structure may be reduced by proper selection of the tilted angle of the substrate. Additionally, the in-plane quantum well density may be increased.
A high quality quantum well structure or quantum wire structure may be used in forming ordered mixed crystal layers. If a two-group compound semiconductor is manufactured by alternately laminating two monomolecular layers at a desired ratio, charge carriers drifting in the non-confinement direction are not subjected to alloy scattering, thus providing a crystal having high mobility characteristics and wherein the mean properties (such as band gap and carrier effective mass) of the mixed crystal are maintained in accordance with the laminated ratio.
As shown in FIG. 5B, the ordered mixed crystal (InAs).sub.1 (GaAs).sub.1 has higher electron mobility than the disordered In.sub.0.5 Ga.sub.0.5 As crystal. This is attributable to the absence of alloy scattering in the ordered mixed crystal.
The electron mobility of the two crystals differs greatly at temperatures near the temperature of liquid nitrogen. The properties of the ordered mixed crystal are therefore superior to the properties of the disordered mixed crystal.